In 1998, Joao Magueijo at Imperial College London, proposed that the speed of light might vary, to solve what cosmologists call the horizon problem. This says that the universe reached a uniform temperature long before heat-carrying photons, which travel at the speed of light, had time to reach all corners of the universe.

The standard way to explain this conundrum is an idea called inflation, which suggests that the universe went through a short period of rapid expansion early on – so the temperature evented out when the cosmos was smaller, then it suddenly grew. But we don’t know why inflation started, or stopped. So Magueijo has been looking for alternatives.

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Now, in a paper to be published 28 November in Physical Review, he and Niayesh Afshordi at the Perimeter Institute in Canada have laid out a new version of the idea – and this one is testable. They suggest that in the early universe, light and gravity propagated at different speeds.

If photons moved faster than gravity just after the big bang, that would have let them get far enough for the universe to reach an equilibrium temperature much more quickly, the team say.

A testable theory

What really excites Magueijo about the idea is that it makes a specific prediction about the cosmic microwave background (CMB). This radiation, which fills the universe, was created shortly after the big bang and contains a “fossilised” imprint of the conditions of the universe.

In Magueijo and Afshordi’s model, certain details about the CMB reflect the way the speed of light and the speed of gravity vary as the temperature of the universe changes. They found that there was an abrupt change at a certain point, when the ratio of the speeds of light and gravity rapidly went to infinity.

This fixes a value called the spectral index, which describes the initial density ripples in the universe, at 0.96478 – a value that can be checked against future measurements. The latest figure, reported by the CMB-mapping Planck satellite in 2015, place the spectral index at about 0.968, which is tantalisingly close.

If more data reveals a mismatch, the theory can be discarded. “That would be great – I won’t have to think about these theories again,” Magueijo says. “This whole class of theories in which the speed of light varies with respect to the speed of gravity will be ruled out.”

But no measurement will rule out inflation entirely, because it doesn’t make specific predictions. “There is a huge space of possible inflationary theories, which makes testing the basic idea very difficult,” says Peter Coles at Cardiff University, UK. “It’s like nailing jelly to the wall.”

That makes it all the more important to explore alternatives like varying light speeds, he adds.

John Webb of the University of New South Wales in Sydney, Australia, has worked for many years on the idea that constants may vary, and is “very impressed” by Magueijo and Afshordi’s prediction. “A testable theory is a good theory,” he says.

The implications could be profound. Physicists have long known there is a mismatch in the way the universe operates on its smallest scales and at its highest energies, and have sought a theory of quantum gravity to unite them. If there is a good fit between Magueijo’s theory and observations, it could bridge this gap, adding to our understanding of the universe’s first moments.

“We have a model of the universe that embraces the idea there must be new physics at some point,” Magueijo says. “It’s complicated, obviously, but I think ultimately there will be a way of informing quantum gravity from this kind of cosmology.”